The present invention relates to power shift transmission and pertains particularly to control means for power shift transmission having a torque converter with a lock-up clutch.
Power shift transmissions having a torque converter in combination with a multi-speed transmission or gear train are frequently used in large vehicles, including off-the-road earth-moving type vehicles. In such transmissions it is very often desirable to provide a direct drive from the engine of the vehicle to the input end of the transmission. This direct drive is often provided by means of a lock-up clutch which is operative to lock the turbine and impeller of the torque converter. This lock-up clutch is provided to be controlled simultaneously with and dependent upon fluid pressure used in the system for the engagement of transmission clutches.
One of the drawbacks of such a system is that fluid pressure for operating clutches on the transmission is often considerably higher than that required for operation of the lock-up clutch. The flow of fluid available from the pressure system is also normally in excess of that which is desirable for operation of a lock-up clutch. This high pressure and high flow rate quite often results in lack of precise control of the engagement of the lock-up clutch.
This high pressure and flow also results in a rapid drop in the system pressure when a valve is open to direct fluid to the lock-up clutch. This quite frequently results in such a drop in pressure that other clutches in the transmission are allowed to slip, resulting in a lack of positive drive through the transmission.
One approach to this problem has been to provide means for controlling the rate of increase of fluid pressure within the lock-up clutch. While this approach may solve the problem of the grabbing of the lock-up clutch, it does not solve the problem of the rapid drop in fluid pressure resulting in disengagement of the other clutches of the transmission.
Therefore, it is desirable to provide a system wherein the lock-up clutch is operated from the same pressure source as that of the rest of the clutches and yet provides smooth and controllable operation.